Lithographic etching of layers of material for subsequent use as either photolithographic, X-ray or e-beam masks, is important, for example, in conjunction with fabricating semiconductor integrated circuits or other devices. Such masks are used for pattern transfer from the mask to the device. Commercially promising layers of material for X-ray and e-beam masks include layers composed of chrome (chromium), chromium-containing compounds, and tungsten. Chrome-containing photolithographic masks are typically a layer of chrome formed on glass or quartz.
In U.S. Pat. No. 4,876,164 issued on Oct. 24, 1989, to Watakabe et al, a method is described for patterning a chrome layer so that it could be used as a mask. In that method a chrome layer is deposited on a transparent glass substrate. An organic resist layer is then formed on the chrome layer, and the resist layer is patterned, as by optical or electron beam scanning (direct writing) followed by wet developing. Then the thus exposed portions of the chrome layer are etched away by means of a gas plasma process using a gaseous mixture that contains oxygen and chlorine. Finally the patterned resist layer is removed.
A problem with the foregoing process, as was found by Watakabe et al, is a low selectivity of the plasma etching of the chrome relative to the etching of the resist. Etch selectivity is the etch rate of one material compared to that of another material under the same etching conditions. Watakabe et al. reports that the ratio of the etch rate of the chrome to that of the resist is undesirably low, typically (as we have found) only about 1.5/1.0 or less when reactive ion etching (RIE) is used. The low selectivity between the resist and the chrome requires the thickness of the resist layer to be undesirably high. When thick resist layers are used during the etching of the chrome, the lateral width of the resist layer is undesirably reduced, whereby the the pattern etched into the chrome layer is degraded.
Jurgensen, C. W., et al., "Tungsten Patterning for 1:1 X-Ray Masks," J. Vac. Sci. Technol., B 9(6), pp. 3320-3286 (1991) describes a process for etching a pattern into a layer of chromium. The chromium layer is formed on a layer of tungsten overlying another layer of chromium which was formed on a polysilicon or silicon nitride membrane. The pattern is formed in the chromium layer by forming an e-beam resist layer onto the top chromium layer, subjecting the resist to a patternwise exposure, and developing the pattern. One of the e-beam resists described in Jurgensen et al. is poly(trimethylsilylmethyl methacrylate-co-3,4-chloromethylstyrene). The pattern is then transferred into the underlying chromium layer using an oxygen/chlorine plasma. The pattern is then transferred into the underlying tungsten layer using a fluorine-based plasma.
Tedesco, S., et al. "Dry Etching For High Resolution Mask Masking" SPIE, Laser Microlithography 111, Vol. 1264, pp. 144, 57 (1990), report that it is difficult to consistently obtain an anisotropic etch profile in chrome using the oxygen-chlorine etch mixture. An anisotropic etch profile is desired for accurate pattern transfer. The more anisotropic the etch profile, i.e., the less lateral etch that occurs, the more accurate the pattern transfer.
Therefore, it would be desirable to have a method for fabricating masks composed of chrome (or of other material that can be anisotropically etched in a gaseous mixture that contains oxygen and chlorine as the etchant species) that does not suffer from the shortcomings of prior art.